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What will swap into a 87 supreme

I have a bone stock 87 cutlass supreme with a 307 y, a th200r4, and a 2.56 open rear end. It has the motive power of a wet sponge. I've seen on other forums that say stuffing in a 3.73 out of a 442 will help tremendously. But, are there other options? Maybe out of a delta? I'm not looking to break the tires but, I know there's more power there than what I'm feeling. I had an 84 delta with the same set up but, 3.23's out back. That one moved out better than my cutlass does. And it was heavier!

Find a 3.73 rear out of another Gbody, perhaps a Monte SS, a Bbody is too wide. Craigslist has them every now and then.
8.5 rear wont do you any good with a 307, unless your drag racing and pushing 400hp at the tires (not the flexplate) a 7.5 will do just fine. Im running a 7.5 3.08 posi with a 375hp, 500ft-lb 455 and the 7.5 is holding up just fine with hard driving.

Find a 3.73 rear out of another Gbody, perhaps a Monte SS, a Bbody is too wide. Craigslist has them every now and then.
8.5 rear wont do you any good with a 307, unless your drag racing and pushing 400hp at the tires (not the flexplate) a 7.5 will do just fine. Im running a 7.5 3.08 posi with a 375hp, 500ft-lb 455 and the 7.5 is holding up just fine with hard driving.

Thanks. That's kind of what I was hoping for. That brings me to another question. Are the GM RPO codes generic? I can get the codes for all the axles used and their ratios. If I were to go to a bone yard and start looking at G Bodies and found a Regal, let's say, with the Codes of GM8 and G89 (2.56 open which my Cutlass has) it would be the same? And at the same token, would I be likely to find other ratios there? Or do you think they would probably be all the same? (ie. most plain jane cutlasses would probably have a 2.56 open) I know these are probably stupid questions but, I'm laid off so money is kinda tight. At the same token, I'm laid off so, I have the time. Thanks!

You just aren't going to beleive this. Just for giggles I went to out local bone yard and sitting there just waiting for me was a RPO code GT4 axle, tube code 2TF out of a '86 Monte SS for $150! Any tips before I stuff it in? Obviously I'm going to strip off all the old crap and clean it up and paint it. But, Should take a look at anything before the actual install while it's off?

I do not trust codes due to people the same as us swapping out parts for various reasons over the years... I generally remove the rear housing covers and divide the ring and pinion teeth counts that are located on the ring gear itself. This is a sure way to know exactly what you are buying. Hope this helps!